The invention provides improved spiral (coiled) or layered capacitors. It is a requirement of various capacitors, for example, low-energy and high-frequency technology capacitors and also capacitors used with alternating current in power grids, that the possible variation in the capacitance should be controlled within a specified range to prevent damage to the capacitor itself and, also, to the installation which includes the capacitor. Aside from a somewhat larger tolerance in the nominal capacitance at start-up, the permitted tolerance (variation) of the capacitance during operation of the capacitor is generally only a few percent. Variation of the capacitance generally results from two different causes. The capacitance may change as a consequence of the change in temperature dependent upon the temperature coefficient of the dielectric contained in the capacitor. Such a capacitance variation is largely reversible. Capacitance also varies during the service life of the capacitor due to aging. This variation is largely irreversible.
The change in capacitance as a consequence of temperature changes can be controlled as is known by selection of the type and amount of solid and liquid insulating components of the capacitor, particularly selecting materials such that the temperature coefficient of the dielectric constant is positive with respect to paper and negative with respect to the liquid impregnating materials. It is also possible to compensate for the capacitance variation resulting from temperature changes by incorporating strips or ribbons of synthetic material in a capacitor to form a composite structure and using synthetics having a temperature coefficient of the dielectric constant which is opposite that of the paper.
Mineral oil is a well-known impregnating material used in capacitors. It is also known to use insulating liquids having a higher dielectric constant, such as dioctylphthalate (DOP) and dinonylphthalate (DNP) in place of mineral oil. The use of these higher dielectric constant impregnating materials is particularly suitable for capacitors having self-healing characteristics. The use of such an impregnating liquid having the higher dielectric constant results in capacitors having a high specific capacitance and high resistance to glow discharge. Tests of such capacitors carried out over lengthy service periods at high temperature and with a high potential, e.g., 100.degree. C. and about 1.3 U.sub.N have produced results reporting that the change in capacitance during the test period was much greater than the variation in capacitance resulting from temperature change. As a consequence, the total change in capacitance during service may exceed the permissible tolerance set by the specifications for the capacitor.
Since such impregnated capacitors which were encapsulated in a housing which also contained a filling medium did not exhibit glow discharge nor decomposition of the impregnating material or of the metallic layers during the extended service test, a possible explanation for the lowering of the capacitance is that the influence of the electric field caused oscillation of the coatings resulting in a larger quantity of insulating material being expelled during the operating period than can be reabsorbed from the reverse flow of material. The interchange of the insulating material is impeded by the swelling of the foil winding which consists of the synthetic material. Oscillations of the coatings caused by pulsation of the electrostatic attractive forces of the alternating field are proportional to the square of the field strength. This explains why there is a small decrease in the capacitance when using dioctylphthalate as the impregnation material when compared with using mineral oil. The dielectric constant of dioctylphthalate is twice the value for mineral oil, resulting in a more advantageous field distribution and a reduced "pumping" action. Following this line of technology, it would be necessary to impregnate and fill a capacitor with a medium having a dielectric constant higher than that of dioctylphthalate in order to obtain a capacitor which exhibits a smaller decreased capacitance during service.